1. Field of the Invention
This invention relates to a packing bag, and more particularly to a double bag structure for packing easy-to-dry moist foods or perishable products.
2. Description of the Related Art
As a packing bag for packing easy-to-dry moist articles or perishable foods, such as fungi including Grifola frondosa and Shimeji (brown button mushrooms), agricultural products such as lettuce, spinach, bean sprouts, fish and seafood, meat and livestock products, there have been extensively used plastic bags made of film of polystyrene, polyethylene polypropylene or the like, which are easy to use, make a fine show and can preserve the freshness of moist foods and suchlike.
Plastic bags used as the packing bag are different in flexibility, ductility, toughness and moisture permeability (moisture permeability) according to the material and thickness thereof. A plastic bag having high moisture permeability could well cause an easy-to-dry moist product such as vegetables to be wilted or parched. On the contrary, a plastic bag having low moisture permeability prevents the moist product from being dried, but entails a disadvantage of forming dew condensation in the bag to mist up the inside of the bag and drench the product in the bag with moisture.
Generally, the plastic bags made of polypropylene film, which have been in heavy usage for packing perishable foods, are sturdy and good moisture retention to prevent the foods from wilting, but suffer from the disadvantage of forming dew in the bag, which gives a foggy appearance of the bag and makes the product in the bag unattractive.
In order to prevent the formation of dew condensation in the bag, breathing holes are sometimes pierced in the polypropylene bag. However, where the breathing holes are made large (e.g. 5 mm in diameter), there is a possibility that an insect or other foreign body may enter the bag through the breathing holes. Conversely, where the breathing holes are made small (e.g. 50 to 100 μm in diameter), they are inconveniently plugged and become ineffective.